Contrail Cloud Radiative Forcing over the Northern Hemisphere Derived from Two Years of MODIS Observations *Douglas A. Spangenberg1, Patrick Minnis2, David P. Duda1, and Sarah T. Bedka1 1Science Systems & Applications, Inc., Hampton, VA

2 NASA-Langley Research Center, Hampton, VA *Corresponding author email: douglas.a.spangenberg@nasa.gov

-2 3. Methodology 2006 Normalized Mean CRF (Wm ) 2012 Total Mean Net CRF (mWm-2) 1. Introduction 2006 2012 • CRF calculated at pixel-level using Fu-Liou radiative SCRF SCRF The constant presence of high-altitude air traffic across the globe influences Earth’s climate transfer model (Fu and Liou 1993; Fu et al., 1998): Day Day through the formation of jet contrails that grow as ice-phase clouds given the right -smooth ice crystal types atmospheric conditions. The contrail radiative forcing (CRF) is used to assess this impact -ice crystal aspect ratio=1.1 on the Earth-Atmosphere system. In this study, satellite data from Aqua and Terra MODIS -2 streams is used to compute the Northern Hemisphere (NH) CRF associated with linear contrails for -data inputs described in section 2. four seasonal months (Jan, Apr Jul, Oct) in the years 2006 and 2012. The contrail cloud properties are retrieved using the NASA-Langley infrared-only Clouds and the Earth’s • CRF=Fconf-Fcon -Fconf = Contrail free upward top-of-atmosphere SW or Radiant Energy System (CERES) algorithm. For each image pixel classified as having a LW flux linear contrail from a sensitive contrail mask, the cloud property retrievals, CERES surface -Fcon = Contrail covered upward top-of-atmosphere SW albedo and emissivity, snow and ice cover, and MERRA atmospheric profiles are used in or LW flux Night Night the 2-stream Fu-Liou radiative transfer model. The CRF is then derived from the model’s LCRF LCRF flux output. The background scene below the contrail is considered to be either clear or • Contrail free, contrail covered fluxes come from same filled with single layer liquid or ice clouds. The shortwave, longwave, and net CRF satellite image pixel in Fu-Liou model. components are computed on a 1x1o grid for both daytime and nighttime conditions and for different background types. The CRF is derived in two ways. For the first method, radiative • Mean values of CRF computed on a 1x1o grid. forcing is normalized to 100% contrail coverage to provide a measure of sensitivity to the presence of contrails. In the second method, the contrail fractions are used to get the total • Normalized CRF: Assume 100% contrail coverage; net forcing which indicates the actual effect on climate. By using actual observations of contrail pixels only are used to get histograms and mean values. Applies to both pixel and gridded data. contrails in their environments, this study indicates the extent to which they affect Earth’s All Data All Data radiation balance. Contrails that become nearly indistinguishable from the background • Total CRF: Uses contrail fraction weighting; takes into cirrus, otherwise known as contrail cirrus, aren’t considered here. account all pixels, with or without contrails. Applies to gridded data. NCRF NCRF

• See Spangenberg et al. (2013) for further details. MODIS 2012 daytime mean cloud phase %, optical 2. Data depth (TAU) for all ice, water clouds (2006 similar CRF Parameter Primary Dependence outside Arctic) • Satellite temporal sampling: tropics, mid-latitudes Shortwave CRF: SCRF -incoming solar radiation sampled 2x/day at ~1:30am, 1:30pm local time Water Cloud % <0 = cooling effect -SFC albedo (Aqua) and ~10:30am, 10:30pm local time (Terra). -cloud albedo [cloud Greater sampling over Arctic. optical depth (τ), cloud particle size (ice=De, Total mean Aqua+Terra MODIS CRF for 2012 (2006); contrail fraction weighting applied • MERRA profile data: T, P, q, O3 water=Re)] - 25 mb vertical res below 700 mb; 50 mb above Longwave CRF: LCRF -SFC skin temperature SCRF (mWm-2) LCRF (mWm-2) NCRF (mWm-2) Contrail 700mb >0 = warming effect - SFC emissivity Pixel mean Aqua+Terra MODIS (normalized) CRF for 2012 (2006) Coverage (%) o - 0.5 x 0.67 horizontal res - cloud τ, cloud altitude SCRF (Wm-2) LCRF (Wm-2) NCRF (Wm-2) N (x107) -View zenith angle Day+Night -4.79 (-7.35) 11.07 (17.92) 6.28 (10.57) 0.099 (0.132) • CERES surface property maps: Albedo, emissivity. Net CRF: NCRF SCRF + LCRF Day+Night -4.93 (-5.72) 11.59 (14.17) 6.66 (8.44) 9.12 (12.31) Day -9.69 (-14.80) 13.06 (19.34) 3.37 (4.54) 0.106 (0.139) Day -9.21 (-10.75) 12.75 (14.50) 3.54 (3.75) 4.88 (6.55) Night 0.0 (0.0) 9.13 (16.53) 9.13 (16.53) 0.092 (0.124) • Snow and ice maps from National Snow & Ice Datacenter using the Interactive Multisensor Mapping Night 0.0 (0.0) 10.25 (13.78) 10.25 (13.78) 4.24 (5.75) 4. Results Pixel mean Aqua+Terra MODIS (normalized) CRF for 2012 (2006) as function of scene type System. Water Cloud τ underneath contrail Normalized CRF Frequency of Occurrence from Pixel Data • Contrail identification from sensitive contrail mask B Water Cloud Ice Cloud Clear of Duda et al. (2013). 2006 Aqua+Terra MODIS 2012 Aqua+Terra MODIS SCRF-Day (Wm-2) -8.34 (-8.65) -8.13 (-10.52) -13.30 (-14.90) • Aqua and Terra MODIS 1-km pixel retrievals from LCRF-Day (Wm-2) 13.40 (14.33) 8.95 (11.63) 16.64 (18.65) CERES/NASA-LaRC algorithm including optical • Primary variable determining icing threat NCRF-Day (Wm-2) 5.02 (5.65) 0.82 (1.11) 3.35 (3.75) depth (τ), pressure, particle size, Tskin (Bedka et al., 2013). Both contrail and background cloud layer Day Day N-Day (x107) 2.63 (2.91) 1.36 (2.08) 0.89 (1.56) properties are used. *NCRF-Night (Wm-2) 12.74 (14.89) 7.55 (10.96) 16.23 (16.75) N-Night (x107) 1.22 (2.96) 2.43 (2.00) 0.59 (0.80)

Ice Cloud % * NCRF=LCRF 5. Summary Preliminary values of MODIS-based contrail radiative forcing over the NH were presented for the seasonal months of the years 2006 and 2012. Highest normalized net CRF occurs with no cloud below the contrail at night (16-17 Wm-2) whereas Night Night the least amount of net forcing occurs in conjunction with ice clouds below the contrail (0.8-1.1 Wm-2). Contrails exhibited the greatest influence over the Sahara Desert with normalized net CRF values reaching 18-20 Wm-2. The overall climatic effect of contrails is shown by the total net CRF; the mean NH values are 10.6 and 6.3 mWm-2 for 2006 and 2012, respectively with values up to 80 mWm-2 (2006) and 60 mWm-2 (2012) over the North Atlantic flight corridor. The decrease in CRF from 2006 to 2012 was primarily due to a decrease in contrail amount and a difference in background cloud *2012 mean contrail amount properties. The differences in CRF between 2006 and 2012 are still under investigation. Ice Cloud τ 6. References Bedka, S. T., P. Minnis, D. P. Duda, R. Boeke, and R. Palikonda (2013), Contrail properties over the Northern Hemisphere from 2006 Aqua

MODIS data, Geophys. Res. Lett., 40, 772-777, doi:10.1029/2012GL054363. Day+Night Day+Night Duda, D. P., P. Minnis, K. Khlopenkov, R. Boeke, and R. Palikonda (2013), Estimation of 2006 Northern Hemisphere contrail coverage using MODIS data, Geophys. Res. Lett., 40, 612-617, doi:10.1002/grl.50097. *2006 similar except increased magnitude Fu, Q. and K.-N. Liou (1993), Parameterization of the radiative properties of cirrus clouds, J. Atmos. Sci., 50, 2008-2025. Fu, Q., P. Yang, and W. B. Sun (1998), An accurate parameterization of the infrared radiative properties of cirrus clouds for climate models, J. Climate, 11, 2223-2237. Spangenberg, D. A., P. Minnis, S. T. Bedka, R. Palikonda, D. P. Duda and F. G. Rose (2013), Contrail radiative forcing over the Northern Hemisphere from 2006 Aqua MODIS data, Geophys. Res. Lett., 40, 595–600, doi:10.1002/grl.50168.